The field of electrophotographic (EP) printers, particularly those intended for an office environment, is actively migrating from mono (single color, i.e., black) printers to color printers. In a known type of color EP printer, four stations associated with four colors, yellow, magenta, cyan, and black, are provided. Each station includes a laser printhead that is scanned to provide a latent image on the charged surface of a photoconductive (PC) drum. The latent image on each drum is developed with the appropriate color toner and transferred onto an intermediate transfer member (ITM) belt. The image is accumulated on the belt by passing each of the four color stations in turn. In another known type of color EP printer, each color image is developed separately on a single PC drum and accumulated on the ITM belt by making four passes past the PC drum. From the ITM belt, the image is transferred to a media substrate such as paper or a transparency. In another known type of color EP printer, a sheet of media is carried on a belt past the four stations and the image is accumulated directly on the media substrate. The toner on the substrate is then fused to the substrate in a fuser assembly, and the substrate is transported out of the printer.
The present invention relates to a single-pass color electrophotographic printer comprising at least two and preferably four imaging stations disposed to define a generally linear path. Preferably, four imaging stations for the toner colors yellow, cyan, magenta, and black are provided. Each imaging station includes an image bearing member, which may be a photoconductive (PC) drum, an optical source such as a laser assembly operative to produce latent images on the image bearing member, a toner source, and a developing member operative to produce developed toned images from the latent image on the image bearing member. An electrically biased first transfer roller is associated with each imaging station. The transfer rollers, which are disposed adjacent to each image bearing member, are operative in conjunction with the image bearing member upon application of the appropriate voltages to transfer toner from the image bearing member to a substrate passing through the nip between the image bearing member and the transfer roller. Servo operations are used to set the operating voltages on each of the rollers at first transfer.
In the present invention, the substrate is an intermediate transfer member (ITM) belt that travels in an endless loop to sequentially contact the image bearing members for transfer of the toned image thereto, in a first transfer operation. In a second transfer operation, the image on the ITM belt is then transferred to the desired media as the belt and the media together pass through a nip between a second electrically biased transfer roller and a backup roller. A servo operation is again used to set the operating voltages on the second transfer roller.
A media guide plate directs media out of the nip between the backup roller and the second transfer roller to a fuser assembly. A media transport belt may be provided between the media guide plate and the fuser assembly if the distance to the fuser assembly is too great for the media to pass unaided.
The ITM belt is supported by at least a first or drive roller and a second or tension roller disposed inside the belt at opposite ends of the generally linear path past the color imaging stations. The backup roller, which is also located on the inside of the belt off the generally linear path, serves as a third support roller for the ITM belt. Preferably, the ITM belt is also supported by a reverse roller downstream of the second transfer operation on the outside of the belt to shift the path of the ITM belt away from the media guide plate and media transport belt.
The ITM belt is formed of a resistive material having a uniform thickness and a high tensile modulus. The bulk resistivity of the belt ranges from 107 to 1012 ohm-cm, preferably about 1010 ohm-cm. The thickness should be as uniform as possible, because variations in thickness cause variations in the velocity of the belt as it travels over the rollers, leading to misregistration of the images on the belt. A nominal thickness of 150 xcexcm has been found to be satisfactory, because this thickness provides adequate tensile strength and can be controlled to within xc2x120 xcexcm, which is an acceptable tolerance.
The color EP printer of the present invention provides robust performance and good color print quality in a single-pass implementation.